Thermionic amplifier



Nov. 8, 19 38. v. M. Cousms 2,135,562

THERM'IONIC AMPLIFIER Filed May 1, 1935 2 Sheets-Sheet 1 [6 a FIG.

#1 R6 s 5 2 Y V V 9L 2/ M/VENTOI? M; cousmls Patented Nov. 8, 1938 P'ATENT OFFICE THERMIONIC AMPLIFIER Van M. Cousins, Lyndhurst, N. J assignor to Bell Telephone Laboratories,- Incorporated, New York, N. Y., a corporation of New York Application May 1, 1935, Serial No. 19,150

2 Claims:

This inven'tionrelates to space discharge vacuumtub'e circuits such'as are employed in signali'n'g systems and sound reproducing systems, fo'rpe'xample, vacuum tube amplifiers or the like.

This invention has as an object the obtaining of an improved fidelity of the reproduction by an amplifier andan increased power output or efficiency', This objectis attained by .the control of modulation products in a manner to be disclosed,

10 -,to increase the power output of fundamental frequency components" While reducing undesired or objectionable harmonics in the amplifier output. A related'object of the invention is to control modulation which takes place in the amplifier ,-,circuit to thereby influence the actual amount of output of the fundamental component as well as the relative amounts of fundamental and harmonic components.

Aieature. of the invention involves a specially go designed circuit in the evenorder current path of an amplifier either in a push-pull stage passing one or more frequencies or in a single tube stage waves ofthe fundamental frequency andwaves :related to the fundamental as harmonics thereof.

Some harmonic frequencies-are quiteobjectionable, particularly the lower order harmonics since'theyare usually the strongest. The output of an amplifier for an impressed complex wave contains'waves of frequencies equal to the sum anddilference of the impressed frequencies and their harmonics; It is usually desirable for the purpose of high quality or'fidelity of reproduction by an amplifier that there be a minimum of modulation products put out by the amplifier.

This invention provides for utilizing the modulation products in increasing the fundamental frequency' power output and in diminishingdistortion'; resulting in greater fidelity of reproduction. The power output in the fundamental frequency is-increased without increase in energy expenditure giving an improved efficiency as well as a better reproduction of the input frequencies to be amplified.

The present invention employs in various embodiments a circuit having suitably disposed resistance and condenser elements of desired values for controlling the even order voltages in the vacuum tube plate circuit and intermodulating these voltages with the fundamental voltage. This invention makes use of the fact that odd order harmonics of the input fundamental frequencies of an amplifier may be generated or limited, or suppressed by intermodulation of the fun- 10 damental frequencies with the even order frequencies. The second order voltages when modulated with the fundamental voltages generate components consisting of new fundamental voltages and third order voltages. These new com ponents are used to increase the output of the fundamental and decrease the output of the third harmonics, orotherwise; as will be described.

The invention will be better understood in illustr'ative embodiments described in the specification and shown in the accompanying drawings 20 in which:

Figs. 1' and 2 are'schematics of a single stage in an amplifier employing the present invention;

Fig. 3' shows characteristic curves for the operation using a mid-branch circuit, as shown in 25 Figs. 1 and 2-;

Fig. 4- shows-a form of circuit especially applicable where a plate supply filter isused.

Fig. 5 is a schematic of a simple arrangement employed for transmitting either a single fre- 30 quency or a very narrow band as in carrier telegraphy'; and

Fig; 6 shows the application of a reversing transformer feed-back arrangement for giving improved operation under certain conditions.

Aswill be'e'xplained more fully hereinafter the invention causes the second order (e. g. double frequency) component to combine with the fundamental to produce some additional useful fundamentaloutp'ut. By such combination or intermodulation it is found possible also to reduce the third order distortion. Thus by reducing the third order distortion and at the same time raising the output of fundamental component, the invention achieves an increased ratio of signal to third order distortion. This is an important achievement, particularly since the reduction of the even order distortion is a comparatively simple matter, as by'using the push- 50 pull type of circuit, which is inherently balanced with respect to even order distortion.

In United States patent to Kreer 1,970,325, August 14, 1934, and in an article by Peterson and Evans entitled Modulation in vacuum tubes used as amplifiers, published in the Bell System Technical Journal, July 1927, it is shown that the coemcient which determines the third order current components is where the 2)- factors are derivatives of the tube static characteristics at the operating point:

1n!n! fi E d E C1 and C2 are respectively the fundamental and second order current coefficients, and R is the load resistance in the plate circuit of the vacuum 7 tube under consideration.

Now in a push-pull amplifier, the load resistance is not the same to all modulation components. It is well-known to those skilled in the art that in such an amplifier, second order voltages are generated in each tube, having such phase relations as to oppose each other in transmitting second order currents around the fundamental frequency path, and therefore the currents are transmitted through the output trans-. former out of phase, and flow back through the plate circuit mid-branch in phase. The load resistance presented to second ordercomponents of each tube is taken as twice the mid-branch circuit impedance. (neglecting the resistance of the output transformer) since the total second tubes instead of one. It can be thought of,

' therefore, as the voltage developed by one of the two tubes in an impedance twice the actual impedance. In a push-pull amplifier, then, the third order coefficient C3 for each tubemay be rewritten to specify the load impedance to each current component, as follows:

to the fourth, second, and direct current order components. Similarly, the third order term C3 contributes'to the third order frequency component, and fundamental frequency component. However, the third order frequency term is totally dependent on the magnitude. of C3, while the third order contribution to the fundamental is only part of the total fundamental energy.

The magnitude of R2 may therefore be selected such that the third order frequency term passes through a minimum at some value, as illustrated in Fig. 3, curves A and B, while the third order contribution to the fundamental frequency has a definite value. As R2 is either increased or decreased from this optimum value for minimum third order component, the third order frequency is a minimum, and the fundamental frequency output has an intermediate 'value, '(3) the third order frequency output is large and the funda-' mental output is a minimum. It should be pointed out that by employing a reversing transformer, and feeding back tothe grid circuit" through this transformer a portion of 'the second order voltage drop developed across the mid- 03"' l l 12+ 1 l 21 1 1 30 2 2 11+ 1 2 1 2 20 V v V i The first four terms of the numerator of this expression involve fundamental frequency volt-' ages and currents, and are therefore not affected by the plate circuit mid-branch impedance. The fifth term may be written as R 0, 512,, R; 6E,

6R, as,

thus producing third order modulation components. Since both the fifth and sixth terms contain R2, which is mainly the plate circuit mid-branch. impedance, it is apparent that the magnitude of these two terms, and consequently the magnitude ofCs will be affected by the midbranch impedance, and that Ca'can be made to branch impedance, R2 may be given'a negative sign, thus extending its .effectivenessbelow the zero value as indicated in brokenlines in Fig." 3.

The circuit of Fig. 1 has input leads 6 and 1 coming from a source of signal energy such as an earlier stage ofan amplifier or a. transmission line and connected'to a transformer 8 the'sec ondary winding 9 of which controls theinput to tubes In and II. The push-pull output circuit-of the tubes In and l l includesthe primary winding M of the transformer 15 and any suitable connection to a load I!) such as a loud-speaker or receiving apparatus.

The power for operating this stage of the amplifier is obtained from the alter nating current source indicated at the primary winding 28 of the transformer.

The output'of. the secondary windings 2| and 22 of the power transformer are usedfor alternatingcurrent filament heating, and the secondary windings .23 furnish the B power which is rectified by the tube 25. The B power obtained from the rectifier 25 is filtered by the inductance L1 in series with the B supply and the condenser C10 and high resistance R3 in shunt thereto. tive side of the B supply circuit of the push-pull The negacircuit passes to the filament through grid bias resistors R5 and. Rs in series to the mid-point of through'filtering resistances: R2 .and.R1..

winding 21.. Resistance R5..- isshunted by; condenser-C12. ofzlow impedance to alternating current. components. The connection to: the grids from; the 1 negative endpf resistance R's extends ResistanceaRzand; condenser Cntogether form a shunt for: alternating current components around the bias. resistors R5. and, Re. any: alternatingv current voltage developed betweemtheterminals; across which condenser; Cu i515 connected is: put back on the. grids. The amount: of thisrvoltage is determined by resistancesz Ri8i1'ldl R6. in series and by the shunting paths around these resistances comprising. Cm; and Rain. series with C11- Therefore, by proportioning elementsCinCiz and R6,..thevo1tage feed-.backican be made zeroror some. finite value. A-lsoby adjusting-these elementsthe impedance of themidx-branch circuit to the alternating currents returning to the filaments can be determined, independently of thetamount of feed-back, inorder'to control the even order modulation products. as previously referred to.

Theci'rcuits of. Fig. .11 were arranged, for purzposes: citest, as shown in'Fig. 2;.to variably shuntresistances Raand-Rs with a tap connecting. to condenser. C12. The characteristic curves of. Fig. 3.show-the varying results in harmonic suppressionand in power. output'control by the changes.

of values. in.the:-circuits ofFig. 2. The circuit of Fig. 2.: employs aB battery source 28 in place of therectifier filter. supply circuitarrangement of Fig- 1. An optimum condition for a minimum thirdzharmonic current is seen, in curve A of. Fig.

3, to exist for a condition of about. 35;ohms resistance. in the mid-branch resistance R6. At this value of mid-branch resistance the power output was nearly a maximum for a condition of no mid-branch feed-back. (Curve C.) With a mid-branch feed-back, the maximum power output is seen to be larger when the resistance Re is smaller. (Curve D.)

Fig. 4 shows a slight rearrangement of the elements of a push-pull output circuit to place certain elements of the mid-branch circuit in better position for a filtering of the B power. In this circuit the condensers C and C12 cooperate with inductance L1 and resistance R5 to form a filter for the plate supply circuit. They also provide a low impedance path from the mid-point of the output primary, respectively, to resistance R2 and resistance Re and in shunt of resistance R5 for the same purpose as explained in connection with Figs. 1 and 2.

The circuit of Fig. 5 adapted for transmitting a single frequency is has leads 3| and 32 coming from a carrier transmission line or an amplifier and through a transformer 33 controlling the grid of a single tube 34. The anode or output circuit of the tube 34 communicates through a transformer 35 to a load 36. The circuit employed for controlling even order products is tuned, as shown, to a frequency 2]0 the second harmonic of the input frequency f0. This control circuit consists of an inductance 38, a condenser 39 and a resistance 40, any or all of which may be variable for purposes of control. Since there is no feed-back in this modification, the curves of Fig. 3 related to mid-branch feed-back are not applicable. However, the advantages obtained with this circuit are analogous to curves representing no feed-back in Fig. 3. The shunt for second order components enables the amount of second harmonic to be controlled and this in turn exerts a control on both the production of third order It is apparent that.

and fundamental components. By. making the shunt of very low impedance to second harmonic, the productionof this component is encouraged and a large amount is therefore available inthe tube for intermodulation with the fundamental in the manner that has been outlined. Depending. on the adjustment of thisshunt in relation to tube characteristic and the impedance of the. rest' of the circuit the fundamental. output may be. increased, the third harmonic decreased, or either or both efiects orxtheir oppositescan be secured. Itis seen fromthe foregoing that this circuit is most effective in the case of a single frequency or when the frequency band transmitted: is sufliciently narrow that. the double frequencies or second harmonics can be effectively transmitted'through the. circuit 38, 39, 40;

The circuits of Fig. 6 are similar to those of Figs. 1 and 2 rearranged as to elements to permit the use of a reversing transformer 42 to provide feed-back permitting a greater power output by effectively giving a reversal of phase of voltage applied'to. the'grids from across the resistance R60, analogous to resistance R6 of Fig. 1, making the magnitude of the resultant voltage impressed on the grids from the mid-branch circuit inversely related to the size of resistance R60. A stopping condenser 43' is in series with the primary winding of this transformer 42 to prevent direct current shunting of one of the biasing resistances R60 corresponding to Rs inFig. 1. mid-branch resistance R50 employed also for biasing resistance hasa functionsimilar tothat of R5 in the schematic of Fig, 1.

Advantages of this invention are shown graphicallyinthe curves. of Fig; 3' whichare the results:;of: measurements: on the. type'of amplifier.-

shown in Fig. 1 or Fig. 2. The measurements on which these curves are based were carried out with only a single constant frequency input at a time. The horizontal ordinates for all curves are in terms of ohms resistance of the mid-branch resistance R6. The vertical ordinates for the power output in watts of the lower group of curves, show, for these two curves, the power output varying between 11 watts and about 14 watts for the two conditions of operation, one with feed-back and the other without feed-back. With mid-branch feed-back and a reversing transformer 42, the power output in watts is seen by the dotted extension of curve D to extend above 14 watts. The middle group of two curves is for third harmonic in terms of decibels below the power of the fundamental. These curves extend from about 22 decibels below fundamental to about 5'7 decibels below fundamental at the minimum point which exists for a condition of no mid-branch feed-back and a resistance of about 35 ohms value for R6. Mid-branch feedback with its advantages is obtained for a positive value of resistance Re by omission of the condenser C11. Where it is desired to extend these curves in the direction of decreasing values of resistance Rs beyond the point where that resistance is zero, this can be done by use of trans former 42, as in Fig. 6, which reverses the phase of the waves developed in resistance R60 before they are applied to the grids. The minimum third harmonic with mid-branch feed-back occurs theoretically when the value of the midbranch resistance in Re is equal to the fraction times the resistance for minimum third harmonic The.

without feed-back. The two curves at the top of Fig. 3 are in milliamperes of second harmonic current flowing in the mid-branch circuit for varying resistance values of the resistance Re i and are seen to vary from a value of about 8 milliamperes to about 30 milliamperes for circuits as in Fig. 1 and greater for mid-branch feedback employing the reversing transformer 42.

An ideal condition has been estimated to exist for a minimum presence of third harmonic current at which point there is a materialimprovement in the power output from the amplifier. By careful consideration of the curves, the condition most desired can be chosen. The power output is seen to be substantially flat above a condition of about 250 ohms value in the resistance R6 for a condition of no mid-branch feedback namely a condition employing the condenser C11 alone without the. reversing transformer 42. The theory to account for the existence of this flat region is somewhat in doubt. The second harmonic current flow decreases slowly as might be expected, with increase in value of resistance R6. The diminution of power of second harmonic current in the mid-branch is seen to be much more rapid, for increases in value of resistance Re, where feed-back is employed than where feed-back is not employed, as is seen from a comparison of the upper two curves of Fig. 3. The decrease should be greater with than without feed-back by a factor of (1+1r). Rs should theoretically be (1+[L) times as effective with mid-branch feed-back as without mid-branch feed-back.

What is claimed is:

1. In a push-pull amplifier stage including thermionic devices having grid, anode and cathode elements, a circuit supplying anode voltage derived from an alternating current source, in- I eluding a filter, a mid-branch circuit with impedance elements including'resistance and condenser elements for controlling transmission to the cathodes of the second order components; said resistance being disposed in series with the anode power supply and said condenser being placed in shunt to the anode'power supply to aid in filtering the anode powersupply, said impedance elements being proportioned to correspond substantially to the minimum point of thecurve of thirdharmonic distortion as a function of midbranch resistance (Re).

2. In a push-pull amplifier stage, a pair of thermionic amplifier tubes having cathode, grid and anode elements, an output transformer con- 7 necting'the anodes of'the tubes, a plate midbranch circuit connecting the mid-point of the output transformer primary to the cathodes of the tubes, a source of anode voltage connected in the plate mid-branch circuit through an in ductance in series with the positive terminal and a first resistance and a second resistance in series with each other and with the-negative terminal,

condensers shunted across the source of anode voltage beyond the inductance and connected to the opposite terminals of said first resistance, said second mentioned resistance being connected to the cathodes, a direct current connection extending from the grids to the negative end of, 

